In a medium saucepan, melt 4 tablespoons of butter over medium heat. Whisk in ¼ cup of all-purpose flour until smooth, cooking for 1-2 minutes. Gradually add 2 cups of milk, stirring until thick, about 5-7 minutes. Add nutmeg, salt, and pepper; set aside.

In a large skillet, heat 2 tablespoons of olive oil over medium heat. Sauté 1 diced onion for 3-4 minutes. Add 2 minced garlic cloves; after 1 minute, stir in 1 cup each of diced red and yellow bell peppers and zucchini. Cook until tender-crisp, about 4-5 minutes. Mix in 1 cup of frozen spinach; season with salt and pepper; cool slightly.

In a large bowl, combine 15 ounces of ricotta cheese, ½ cup of grated Parmesan cheese, and 1 beaten egg. Add the cooled vegetable mixture; stir until well incorporated.

Prepare 9 lasagna noodles according to package instructions, boiling for 8-10 minutes until al dente. Drain and rinse under cold water to prevent sticking.

In a 9x13 inch baking dish, spread a layer of white sauce on the bottom. Layer 3 lasagna noodles, half of the vegetable filling, and 1 cup of shredded mozzarella cheese. Repeat the layers, finishing with noodles, white sauce, and remaining mozzarella on top.

Cover with foil and bake in a preheated oven at 375°F (190°C) for 25 minutes. Remove foil and bake for an additional 20-25 minutes until cheese is melted and bubbly. Let rest for 10 minutes before serving.